1. Field of the Invention
The present invention relates to a method of producing a semiconductor laser diode used mainly in optical communications and particularly an embedded double junction semiconductor laser diode based on InP and that has a current blocking layer.
2. Description of the Related Art
Semiconductor laser diodes that have embedded layers based on InP on both sides of a mesa have been used as the light source for optical communications. FIGS. 26A-26D are schematic diagrams for explaining the respective steps in the production of the semiconductor laser diode of a conventional example (hereinafter referred to as a first conventional example). The semiconductor laser diode of the conventional example has been produced by forming an n cladding layer 2 made of n-InP, an active layer 3 and a p cladding layer 4 made of p-InP successively on a substrate 1 made of n-InP as shown in FIG. 26A. Thereafter, as shown in FIG. 26B, an oxide film 205 is formed on a portion where a mesa is to be formed, and the other portion is etched away thereby forming the mesa. During this etching step, so-called undercut occurs below both edges of the oxide film 205, causing the edges of the oxide film 205 to project in a manner of overhang on both sides.
Then a p-InP layer 206 and an n-InP layer 207 are grown on both sides of the mesa as shown in FIG. 26C, and the oxide film 205 is removed while forming a p contact layer 8 and a contact layer 9 as shown in FIG. 26D. The semiconductor laser diode of the first conventional example thus produced has a problem of a current leakage path 210 being formed as shown in FIG. 26D that allows current leakage. A method of producing a semiconductor laser diode (hereinafter referred to as a second conventional example) that minimizes the current leakage is disclosed, for example, in the Japanese Patent No. 2828628. The production method disclosed in the Japanese Patent No. 2828628 comprises forming the n cladding layer 2 made of n-InP, the active layer 3 and the p cladding layer 4 made of p-InP successively on the substrate 1 made of n-InP as shown in FIG. 27A, followed by the formation of a nitride film 211 and an oxide film 212 on the p cladding layer 4 that is made of p-InP.
Then as shown in FIG. 27B, a photosensitive film 213 is formed on a portion where a mesa is to be formed, and the nitride film 211 and the oxide film 212 are etched away by using the photosensitive film 213 as a mask in a wet etching step using hydrofluoric acid, thereby forming a nitride film 211a and an oxide film 212a. In this wet etching step, since the etching rate is higher for the oxide film than for the nitride film, the oxide film 211a becomes smaller than the nitride film 212a as shown in FIG. 27B. Then as shown in FIG. 27C, the photosensitive film 213 is removed and the mesa is formed by etching with the nitride film 211a and the oxide film 212a as the mask. During this etching step, undercut occurs below both edges of the nitride film 211a, making the nitride film 211a wider than the width of the mesa as shown in FIG. 27A, and therefore protruding edges are formed similarly to the first conventional example.
Then as shown in FIG. 28A, the protruding edges of the nitride film 211a are removed through dry etching by making use of the fact that the etching rate is higher for the nitride 211a film than for the oxide film 212a in dry etching. Thereafter, as shown in FIG. 28B, a p-InP layer 214 and an n-InP layer 215 are grown on both sides of the mesa, followed by removal of the oxide film 212a and the nitride film 211a thereby to form a p contact layer 216 and a contact layer 217 as shown in FIG. 28C. In the semiconductor laser diode of the second conventional example thus produced, leakage current is less than that in the semiconductor laser diode of the first conventional example.
However, the semiconductor laser diode of the first conventional example had such a problem that the leakage current cannot be reduced as described above.
There was also a problem that an active layer is exposed on side faces of the mesa during dry etching of the protruding edges of the nitride film, resulting in a possibility of lowering the reliability of the active layer 3.